The synthesis of diamond crystals by high temperature, high pressure processes are well known. Preferred methods for making diamonds are disclosed in Hall et al., U.S. Pat. No. 2,947,610, Strong, U.S. Pat. No. 2,947,609. Apparatus for carrying out such processes is described in Hall, U.S. Pat. No. 2,941,248.
Briefly, diamond growth in the aforementioned processes occurs by the diffusion of carbon through a thin metallic film of any of a series of specific catalyst, preferably an alloy of at least two metals with one of the metals selected from the class consisting of group VIII metals of the periodic table, chromium, tantalum and manganese. The reaction results in a composition, referred to as a "run," which contains a mixture of diamond; graphite (from the carbon source); and the catalyst.
The recovery of the diamond from the run has presented certain problems relating to cost and undesirable by-products. The recovery of diamonds from the runs has generally been accomplished by the treatment of the runs with acids. However, in these acid recovery processes, treatment of ventilation air and waste acids is necessary prior to discharge from the recovery facility into waste disposal facilities.
Therefore, the acid recovery process requires substantial capital expenditures to limit pollution discharge, and results in wasted material.
Additionally, drawbacks have been observed in all of these processes. A complete removal of graphite often requires repeating the process several times in the acid methods, thereby resulting in time consumption and material loss in addition to the pollution problems mentioned above.
Finally, complete removal of the graphite or non-diamond carbonaceous material is difficult when small amounts of graphite are left after removal of substantially all the other synthesis material from the diamond crystals. The final removal of graphite oxidation of the graphite, utilizing alkali earth metal carbonates has been suggested, see Showa Denko KK, Japanese Pat. No. SHO 47-44159, issued Nov. 8, 1972. However, some diamond loss due to oxidation is inevitable.
The synthesis of diamond crystals may also be achieved by a dynamic process as opposed to the so-called static processes just described. An example of such a dynamic process is described in Balchan, U.S. Pat. No. 3,608,014 wherein an explosive charge is utilized to create the pressure and temperature necessary for the conversion of graphite into diamond.
The recovery of the diamond crystals from a dynamic process presents similar problems to those encountered with static processes and as described in Example I of the Balcan patent an oxidation process is used to separate the graphite and diamonds which leads to undesirable polution problems.